This invention relates to an apparatus and technique for determining changes in the liquid level of a storage tank with extreme accuracy.
Many liquid petroleum products are stored in very large quantities, frequently in underground tanks. One concern with underground liquid storage is the possibility that the tank may develop a leak which is not observable on the surface. It is a special concern when the leak is one which is relatively minor over a short period of time but which left undetected and unrepaired can result in very substantial leakage over a long period of time.
Because of concerns of petroleum products and other liquids escaping into ground water aquifers, government regulations require strict monitoring of underground storage tanks and provisions to detect even minor leaks as quickly as possible. Since visual inspection is not possible in underground storage, leakage detection must depend upon very accurate measurements of the liquid level in the tank from one day to another to ensure that a small quantity of the liquid is not leaking out of the tank over time.
The problem is compounded by the fact that underground tanks hold a large volume of liquid and that a small leak will not result in a significant change in the level of liquid in the large tank. For that reason a small leak is extremely difficult to measure. One such technique which has been developed is illustrated in U.S. Pat. No. 4,182,178 which discloses the use of a sensitive manometer to detect changes in the surface level of the liquid. While this technique is useful with some kinds of petroleum products, a problem arises when attempting to use the technique with gasoline products containing alcohol. Typically the manometer fluid used is a mixture of alcohol and water proportioned to have a precise specific gravity relevant to the gasoline in the tank. When the gasoline itself contains alcohol the intermingling of the manometer fluid and the gasoline/alcohol mixture destroys the predictability and accuracy of the measuring system.